BUG/MAJOR: queue: better protect a pendconn being picked from the proxy
The locking in the dequeuing process was significantly improved by commit
49667c14b ("MEDIUM: queue: take the proxy lock only during the px queue
accesses") in that it tries hard to limit the time during which the
proxy's queue lock is held to the strict minimum. Unfortunately it's not
enough anymore, because we take up the task and manipulate a few pendconn
elements after releasing the proxy's lock (while we're under the server's
lock) but the task will not necessarily hold the server lock since it may
not have successfully found one (e.g. timeout in the backend queue). As
such, stream_free() calling pendconn_free() may release the pendconn
immediately after the proxy's lock is released while the other thread
currently proceeding with the dequeuing tries to wake up the owner's
task and dies in task_wakeup().
One solution consists in releasing le proxy's lock later. But tests have
shown that we'd have to sacrifice a significant share of the performance
gained with the patch above (roughly a 20% loss).
This patch takes another approach. It adds a "del_lock" to each pendconn
struct, that allows to keep it referenced while the proxy's lock is being
released. It's mostly a serialization lock like a refcount, just to maintain
the pendconn alive till the task_wakeup() call is complete. This way we can
continue to release the proxy's lock early while keeping this one. It had
to be added to the few points where we're about to free a pendconn, namely
in pendconn_dequeue() and pendconn_unlink(). This way we continue to
release the proxy's lock very early and there is no performance degradation.
This lock may only be held under the queue's lock to prevent lock
inversion.
No backport is needed since the patch above was merged in 2.5-dev only.
2 files changed