'Twas brillig, and David Henningsson at 19/08/11 09:21 did gyre and gimble: > Hi, > > I came across this bug: > > https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/817269 > > Which claims the following (and also offers a patch [1] to fix it). I > don't know much about file locking or NFS, but maybe someone here knows > more than I here. > > Quoting the bug: > > Pulseaudio uses a cookie file (normally ~/.pulse-cookie). This file is > manipulated by the code in src/pulsecore/authkey.c. Currently it does > this to read the key: > 1) Open the file > 2) Acquire a write-lock for the file. > 3) Read the file. > 4) If the file is not a good cookie, generate a new cookie and write it > to the file. > 5) Close the file. > > If more than one thread is opening the cookie at once, then there is > contention at step (2) which is rarely necessary. Instead, step (2) > should acquire only a read-lock, and then step (4) should promote that > to a write-lock before writing the file. > > This is particularly bad in the case where the .pulse-cookie file is on > NFS. In that case, the lock contention forces a thirty-second backoff > from whichever thread gets there second. > > I am happy to work with y'all in engineering a patch for this. It > requires obviously a change to src/pulsecore/authkey.c, and a > corresponding one in src/pulsecore/core-util.c where the actual lock > syscall occurs. > > The following script (if ~ is in NFS) will demonstrate the problem. The > two pactl processes both try to acquire the lock on the file > simultaneously, and whichever one loses will take thirty seconds before > it wins, because of the needless contention (and the unfortunate facts > of how lockd in NFS works). > > !/bin/bash > > NUM_PROCS=2 > > procs=() > for i in $(seq 1 $NUM_PROCS); do > pactl list > /dev/null & > procs[$i]=$! > done > for i in $(seq 1 $NUM_PROCS); do > wait ${procs[$i]} > done > > On first glance, this seems sensible, but I'd like Lennart's comment on this one as locking is one area I don't feel super confident playing around with! Lennart, thoughts? See the link for the proposed patch. Col -- Colin Guthrie gmane(at)colin.guthr.ie http://colin.guthr.ie/ Day Job: Tribalogic Limited [http://www.tribalogic.net/] Open Source: Mageia Contributor [http://www.mageia.org/] PulseAudio Hacker [http://www.pulseaudio.org/] Trac Hacker [http://trac.edgewall.org/]
