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nptl: Remove CANCELING_BITMASK

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The CANCELING_BITMASK is used as an optimization to avoid sending
the signal when pthread_cancel is called in a concurrent manner.

This requires then to put both the cancellation state and type on a
shared state (cancelhandling), since 'pthread_cancel' checks whether
cancellation is enabled and asynchrnous to either cancel itself of
 sending the signal.

It also requires handle the CANCELING_BITMASK on
__pthread_disable_asynccancel, however this incurs in the same issues
described on BZ#12683: the cancellation is acted upon even *after*
syscall returns with user visible side-effects.

This patch removes this optimization and simplifies the pthread
cancellation implementation: pthread_cancel now first checks if
cancellation is already pending and if not always, sends a signal
if the target is not itself.  The SIGCANCEL handler is also simpified
since there is not need to setup a CAS loop.

It also allows to move both the cancellation state and mode out of
'cancelhadling' (it is done in subsequent patches).

Checked on x86_64-linux-gnu and aarch64-linux-gnu.
This commit is contained in:
Adhemerval Zanella 2021-05-25 14:31:30 -03:00
parent 41c7295617
commit 26cfbb7162
4 changed files with 36 additions and 106 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
nptl/cancellation.c
View File

@ -88,17 +88,5 @@ __pthread_disable_asynccancel (int oldtype)
/* Prepare the next round. */
oldval = curval;
}
/* We cannot return when we are being canceled. Upon return the
thread might be things which would have to be undone. The
following loop should loop until the cancellation signal is
delivered. */
while (__builtin_expect ((newval & (CANCELING_BITMASK | CANCELED_BITMASK))
== CANCELING_BITMASK, 0))
{
futex_wait_simple ((unsigned int *) &self->cancelhandling, newval,
FUTEX_PRIVATE);
newval = THREAD_GETMEM (self, cancelhandling);
}
}
libc_hidden_def (__pthread_disable_asynccancel)

3
nptl/descr.h
View File

@ -283,9 +283,6 @@ struct pthread
/* Bit set if asynchronous cancellation mode is selected. */
#define CANCELTYPE_BIT 1
#define CANCELTYPE_BITMASK (0x01 << CANCELTYPE_BIT)
/* Bit set if canceling has been initiated. */
#define CANCELING_BIT 2
#define CANCELING_BITMASK (0x01 << CANCELING_BIT)
/* Bit set if canceled. */
#define CANCELED_BIT 3
#define CANCELED_BITMASK (0x01 << CANCELED_BIT)

125
nptl/pthread_cancel.c
View File

@ -43,35 +43,18 @@ sigcancel_handler (int sig, siginfo_t *si, void *ctx)
struct pthread *self = THREAD_SELF;
int oldval = THREAD_GETMEM (self, cancelhandling);
while (1)
{
/* We are canceled now. When canceled by another thread this flag
is already set but if the signal is directly send (internally or
from another process) is has to be done here. */
int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
int ch = atomic_load_relaxed (&self->cancelhandling);
/* Cancelation not enabled, not cancelled, or already exitting. */
if ((ch & CANCELSTATE_BITMASK) != 0
|| (ch & CANCELED_BITMASK) == 0
|| (ch & EXITING_BITMASK) != 0)
return;
if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
/* Already canceled or exiting. */
break;
int curval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling, newval,
oldval);
if (curval == oldval)
{
/* Set the return value. */
THREAD_SETMEM (self, result, PTHREAD_CANCELED);
/* Make sure asynchronous cancellation is still enabled. */
if ((newval & CANCELTYPE_BITMASK) != 0)
/* Run the registered destructors and terminate the thread. */
__do_cancel ();
break;
}
oldval = curval;
}
/* Set the return value. */
THREAD_SETMEM (self, result, PTHREAD_CANCELED);
/* Make sure asynchronous cancellation is still enabled. */
if ((ch & CANCELTYPE_BITMASK) != 0)
__do_cancel ();
}
int
@ -104,72 +87,34 @@ __pthread_cancel (pthread_t th)
" must be installed for pthread_cancel to work\n");
}
#endif
int result = 0;
int oldval;
int newval;
do
int oldch = atomic_fetch_or_acquire (&pd->cancelhandling, CANCELED_BITMASK);
if ((oldch & CANCELED_BITMASK) != 0)
return 0;
if (pd == THREAD_SELF)
{
again:
oldval = pd->cancelhandling;
newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
/* Avoid doing unnecessary work. The atomic operation can
potentially be expensive if the bug has to be locked and
remote cache lines have to be invalidated. */
if (oldval == newval)
break;
/* If the cancellation is handled asynchronously just send a
signal. We avoid this if possible since it's more
expensive. */
if (CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS (newval))
{
/* Mark the cancellation as "in progress". */
if (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling,
oldval | CANCELING_BITMASK,
oldval))
goto again;
if (pd == THREAD_SELF)
/* This is not merely an optimization: An application may
call pthread_cancel (pthread_self ()) without calling
pthread_create, so the signal handler may not have been
set up for a self-cancel. */
{
THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
if ((newval & CANCELTYPE_BITMASK) != 0)
__do_cancel ();
}
else
{
/* The cancellation handler will take care of marking the
thread as canceled. */
pid_t pid = __getpid ();
int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid,
SIGCANCEL);
if (INTERNAL_SYSCALL_ERROR_P (val))
result = INTERNAL_SYSCALL_ERRNO (val);
}
break;
}
/* A single-threaded process should be able to kill itself, since
there is nothing in the POSIX specification that says that it
cannot. So we set multiple_threads to true so that cancellation
points get executed. */
THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
/* A single-threaded process should be able to kill itself, since there
is nothing in the POSIX specification that says that it cannot. So
we set multiple_threads to true so that cancellation points get
executed. */
THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
#ifndef TLS_MULTIPLE_THREADS_IN_TCB
__libc_multiple_threads = 1;
__libc_multiple_threads = 1;
#endif
}
/* Mark the thread as canceled. This has to be done
atomically since other bits could be modified as well. */
while (atomic_compare_and_exchange_bool_acq (&pd->cancelhandling, newval,
oldval));
return result;
THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
if ((oldch & CANCELSTATE_BITMASK) == 0
&& (oldch & CANCELTYPE_BITMASK) != 0)
__do_cancel ();
return 0;
}
pid_t pid = __getpid ();
int val = INTERNAL_SYSCALL_CALL (tgkill, pid, pd->tid, SIGCANCEL);
return INTERNAL_SYSCALL_ERROR_P (val)
? INTERNAL_SYSCALL_ERRNO (val)
: 0;
}
versioned_symbol (libc, __pthread_cancel, pthread_cancel, GLIBC_2_34);

2
nptl/pthread_join_common.c
View File

@ -57,7 +57,7 @@ __pthread_clockjoin_ex (pthread_t threadid, void **thread_return,
if ((pd == self
|| (self->joinid == pd
&& (pd->cancelhandling
& (CANCELING_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
& (CANCELED_BITMASK | EXITING_BITMASK
| TERMINATED_BITMASK)) == 0))
&& !CANCEL_ENABLED_AND_CANCELED (self->cancelhandling))
/* This is a deadlock situation. The threads are waiting for each