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cifs: prevent starvation in wait_for_free_credits for multi-credit requests

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Reserve the last MAX_COMPOUND credits for any request asking for >1 credit.
This is to prevent future compound requests from becoming starved while waiting
for potentially many requests is there is a large number of concurrent
singe-credit requests.

However, we need to protect from servers that are very slow to hand out
new credits on new sessions so we only do this IFF there are 2*MAX_COMPOUND
(arbitrary) credits already in flight.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
This commit is contained in:
Ronnie Sahlberg 2019-03-08 12:58:21 +10:00 committed by Steve French
parent b227d215de
commit 16b34aa44b
1 changed files with 28 additions and 0 deletions
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28
fs/cifs/transport.c
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@ -528,6 +528,34 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
return -ENOENT;
}
/*
* For normal commands, reserve the last MAX_COMPOUND
* credits to compound requests.
* Otherwise these compounds could be permanently
* starved for credits by single-credit requests.
*
* To prevent spinning CPU, block this thread until
* there are >MAX_COMPOUND credits available.
* But only do this is we already have a lot of
* credits in flight to avoid triggering this check
* for servers that are slow to hand out credits on
* new sessions.
*/
if (!optype && num_credits == 1 &&
server->in_flight > 2 * MAX_COMPOUND &&
*credits <= MAX_COMPOUND) {
spin_unlock(&server->req_lock);
cifs_num_waiters_inc(server);
rc = wait_event_killable(server->request_q,
has_credits(server, credits,
MAX_COMPOUND + 1));
cifs_num_waiters_dec(server);
if (rc)
return rc;
spin_lock(&server->req_lock);
continue;
}
/*
* Can not count locking commands against total
* as they are allowed to block on server.