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[Linux] Optimize PID -> struct lwp_info lookup
Hacking the gdb.threads/attach-many-short-lived-threads.exp test to
spawn thousands of threads instead of dozens, and running gdb under
perf, I saw that GDB was spending most of the time in find_lwp_pid:
- captured_main
- 93.61% catch_command_errors
- 87.41% attach_command
- 87.40% linux_nat_attach
- 87.40% linux_proc_attach_tgid_threads
- 82.38% attach_proc_task_lwp_callback
- 81.01% find_lwp_pid
5.30% ptid_get_lwp
+ 0.10% ptid_lwp_p
+ 0.64% add_thread
+ 0.26% set_running
+ 0.24% set_executing
0.12% ptid_get_lwp
+ 0.01% ptrace
+ 0.01% add_lwp
attach_proc_task_lwp_callback is called once for each LWP that we
attach to, found by listing the /proc/PID/task/ directory. In turn,
attach_proc_task_lwp_callback calls find_lwp_pid to check whether the
LWP we're about to try to attach to is already known. Since
find_lwp_pid does a linear walk over the whole LWP list, this becomes
quadratic. We do the /proc/PID/task/ listing until we get two
iterations in a row where we found no new threads. So the second and
following times we walk the /proc/PID/task/ dir, we're going to take
an even worse find_lwp_pid hit.
Fix this by adding a hash table keyed by LWP PID, for fast lookup.
The linked list embedded in the LWP structure itself is kept, and made
a double-linked list, so that removals from that list are O(1). An
earlier version of this patch got rid of this list altogether, but
that revealed hidden dependencies / assumptions on how the list is
sorted. For example, killing a process and then waiting for all the
LWPs status using iterate_over_lwps only works as is because the
leader LWP is always last in the list. So I thought it better to take
an incremental approach and make this patch concern itself _only_ with
the PID lookup optimization.
gdb/ChangeLog:
2016-05-24 Pedro Alves <palves@redhat.com>
PR gdb/19828
* linux-nat.c (lwp_lwpid_htab): New htab.
(lwp_info_hash, lwp_lwpid_htab_eq, lwp_lwpid_htab_create)
(lwp_lwpid_htab_add_lwp): New functions.
(lwp_list): Tweak comment.
(lwp_list_add, lwp_list_remove, lwp_lwpid_htab_remove_pid): New
functions.
(purge_lwp_list): Rewrite, using htab_traverse_noresize.
(add_initial_lwp): Add lwp to htab too. Use lwp_list_add.
(delete_lwp): Use lwp_list_remove. Remove htab too.
(find_lwp_pid): Search in htab.
(_initialize_linux_nat): Call lwp_lwpid_htab_create.
* linux-nat.h (struct lwp_info) <prev>: New field.
This commit is contained in:
Pedro Alves
parent
1ad3de988d
commit
774113b02f
@ -1,3 +1,35 @@
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2016-05-24 Pedro Alves <palves@redhat.com>
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PR gdb/19828
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* linux-nat.c (lwp_lwpid_htab): New htab.
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(lwp_info_hash, lwp_lwpid_htab_eq, lwp_lwpid_htab_create)
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(lwp_lwpid_htab_add_lwp): New functions.
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(lwp_list): Tweak comment.
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(lwp_list_add, lwp_list_remove, lwp_lwpid_htab_remove_pid): New
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functions.
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(purge_lwp_list): Rewrite, using htab_traverse_noresize.
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(add_initial_lwp): Add lwp to htab too. Use lwp_list_add.
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(delete_lwp): Use lwp_list_remove. Remove htab too.
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(find_lwp_pid): Search in htab.
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(_initialize_linux_nat): Call lwp_lwpid_htab_create.
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* linux-nat.h (struct lwp_info) <prev>: New field.
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2016-05-24 Pedro Alves <palves@redhat.com>
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PR gdb/19828
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* linux-nat.c (lwp_lwpid_htab): New htab.
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(lwp_info_hash, lwp_lwpid_htab_eq, lwp_lwpid_htab_create)
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(lwp_lwpid_htab_add_lwp): New functions.
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(lwp_list): Tweak comment.
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(lwp_list_add, lwp_list_remove, lwp_lwpid_htab_remove_pid): New
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functions.
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(purge_lwp_list): Rewrite, using htab_traverse_noresize.
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(add_initial_lwp): Add lwp to htab too. Use lwp_list_add.
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(delete_lwp): Use lwp_list_remove. Remove htab too.
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(find_lwp_pid): Search in htab.
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(_initialize_linux_nat): Call lwp_lwpid_htab_create.
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* linux-nat.h (struct lwp_info) <prev>: New field.
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2016-05-24 Pedro Alves <palves@redhat.com>
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PR gdb/19828
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163
gdb/linux-nat.c
163
gdb/linux-nat.c
@ -677,8 +677,86 @@ linux_child_set_syscall_catchpoint (struct target_ops *self,
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return 0;
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}
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/* List of known LWPs. */
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/* List of known LWPs, keyed by LWP PID. This speeds up the common
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case of mapping a PID returned from the kernel to our corresponding
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lwp_info data structure. */
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static htab_t lwp_lwpid_htab;
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/* Calculate a hash from a lwp_info's LWP PID. */
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static hashval_t
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lwp_info_hash (const void *ap)
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{
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const struct lwp_info *lp = (struct lwp_info *) ap;
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pid_t pid = ptid_get_lwp (lp->ptid);
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return iterative_hash_object (pid, 0);
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}
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/* Equality function for the lwp_info hash table. Compares the LWP's
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PID. */
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static int
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lwp_lwpid_htab_eq (const void *a, const void *b)
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{
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const struct lwp_info *entry = (const struct lwp_info *) a;
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const struct lwp_info *element = (const struct lwp_info *) b;
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return ptid_get_lwp (entry->ptid) == ptid_get_lwp (element->ptid);
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}
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/* Create the lwp_lwpid_htab hash table. */
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static void
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lwp_lwpid_htab_create (void)
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{
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lwp_lwpid_htab = htab_create (100, lwp_info_hash, lwp_lwpid_htab_eq, NULL);
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}
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/* Add LP to the hash table. */
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static void
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lwp_lwpid_htab_add_lwp (struct lwp_info *lp)
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{
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void **slot;
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slot = htab_find_slot (lwp_lwpid_htab, lp, INSERT);
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gdb_assert (slot != NULL && *slot == NULL);
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*slot = lp;
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}
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/* Head of doubly-linked list of known LWPs. Sorted by reverse
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creation order. This order is assumed in some cases. E.g.,
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reaping status after killing alls lwps of a process: the leader LWP
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must be reaped last. */
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struct lwp_info *lwp_list;
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/* Add LP to sorted-by-reverse-creation-order doubly-linked list. */
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static void
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lwp_list_add (struct lwp_info *lp)
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{
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lp->next = lwp_list;
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if (lwp_list != NULL)
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lwp_list->prev = lp;
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lwp_list = lp;
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}
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/* Remove LP from sorted-by-reverse-creation-order doubly-linked
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list. */
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static void
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lwp_list_remove (struct lwp_info *lp)
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{
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/* Remove from sorted-by-creation-order list. */
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if (lp->next != NULL)
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lp->next->prev = lp->prev;
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if (lp->prev != NULL)
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lp->prev->next = lp->next;
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if (lp == lwp_list)
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lwp_list = lp->next;
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}
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/* Original signal mask. */
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@ -754,31 +832,30 @@ lwp_free (struct lwp_info *lp)
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xfree (lp);
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}
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/* Traversal function for purge_lwp_list. */
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static int
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lwp_lwpid_htab_remove_pid (void **slot, void *info)
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{
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struct lwp_info *lp = (struct lwp_info *) *slot;
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int pid = *(int *) info;
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if (ptid_get_pid (lp->ptid) == pid)
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{
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htab_clear_slot (lwp_lwpid_htab, slot);
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lwp_list_remove (lp);
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lwp_free (lp);
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}
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return 1;
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}
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/* Remove all LWPs belong to PID from the lwp list. */
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static void
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purge_lwp_list (int pid)
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{
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struct lwp_info *lp, *lpprev, *lpnext;
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lpprev = NULL;
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for (lp = lwp_list; lp; lp = lpnext)
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{
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lpnext = lp->next;
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if (ptid_get_pid (lp->ptid) == pid)
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{
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if (lp == lwp_list)
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lwp_list = lp->next;
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else
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lpprev->next = lp->next;
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lwp_free (lp);
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}
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else
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lpprev = lp;
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}
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htab_traverse_noresize (lwp_lwpid_htab, lwp_lwpid_htab_remove_pid, &pid);
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}
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/* Add the LWP specified by PTID to the list. PTID is the first LWP
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@ -812,8 +889,11 @@ add_initial_lwp (ptid_t ptid)
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lp->ptid = ptid;
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lp->core = -1;
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lp->next = lwp_list;
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lwp_list = lp;
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/* Add to sorted-by-reverse-creation-order list. */
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lwp_list_add (lp);
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/* Add to keyed-by-pid htab. */
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lwp_lwpid_htab_add_lwp (lp);
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return lp;
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}
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@ -844,22 +924,24 @@ add_lwp (ptid_t ptid)
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static void
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delete_lwp (ptid_t ptid)
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{
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struct lwp_info *lp, *lpprev;
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struct lwp_info *lp;
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void **slot;
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struct lwp_info dummy;
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lpprev = NULL;
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for (lp = lwp_list; lp; lpprev = lp, lp = lp->next)
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if (ptid_equal (lp->ptid, ptid))
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break;
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if (!lp)
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dummy.ptid = ptid;
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slot = htab_find_slot (lwp_lwpid_htab, &dummy, NO_INSERT);
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if (slot == NULL)
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return;
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if (lpprev)
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lpprev->next = lp->next;
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else
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lwp_list = lp->next;
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lp = *(struct lwp_info **) slot;
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gdb_assert (lp != NULL);
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htab_clear_slot (lwp_lwpid_htab, slot);
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/* Remove from sorted-by-creation-order list. */
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lwp_list_remove (lp);
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/* Release. */
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lwp_free (lp);
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}
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@ -871,17 +953,16 @@ find_lwp_pid (ptid_t ptid)
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{
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struct lwp_info *lp;
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int lwp;
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struct lwp_info dummy;
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if (ptid_lwp_p (ptid))
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lwp = ptid_get_lwp (ptid);
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else
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lwp = ptid_get_pid (ptid);
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for (lp = lwp_list; lp; lp = lp->next)
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if (lwp == ptid_get_lwp (lp->ptid))
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return lp;
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return NULL;
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dummy.ptid = ptid_build (0, lwp, 0);
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lp = (struct lwp_info *) htab_find (lwp_lwpid_htab, &dummy);
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return lp;
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}
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/* See nat/linux-nat.h. */
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@ -4861,6 +4942,8 @@ Enables printf debugging output."),
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sigdelset (&suspend_mask, SIGCHLD);
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sigemptyset (&blocked_mask);
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lwp_lwpid_htab_create ();
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}
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@ -101,7 +101,9 @@ struct lwp_info
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/* Arch-specific additions. */
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struct arch_lwp_info *arch_private;
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/* Next LWP in list. */
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/* Previous and next pointers in doubly-linked list of known LWPs,
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sorted by reverse creation order. */
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struct lwp_info *prev;
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struct lwp_info *next;
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};
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