On Thu, 27 Feb 2014 13:51:25 -0800 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Fri, 28 Feb 2014 08:34:43 +1100 NeilBrown <neilb@xxxxxxx> wrote: > > > On Thu, 27 Feb 2014 12:58:07 -0800 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > wrote: > > > > > On Thu, 27 Feb 2014 17:24:45 +1100 NeilBrown <neilb@xxxxxxx> wrote: > > > > > > > If poll or select is waiting on /proc/mdstat when md-mod is unloaded > > > > an oops will ensure when the poll/select completes. > > > > > > > > This is because the wait_queue_head which is registered with poll_wait() > > > > is local to the module and no longer exists when the poll completes and > > > > detaches that wait_queue_head (in poll_free_wait -> remove_wait_queue). > > > > > > > > To fix this we need the wait_queue_head to have (at least) the same life > > > > time as the proc_dir_entry. So this patch places it in that structure. > > > > > > > > We: > > > > - add pde_poll_wait to struct proc_dir_entry > > > > - call poll_wait() passing this when poll() is called on the proc file > > > > - export a function proc_wake_up which will call wake_up() on pde_poll_wait > > > > > > > > and make use of all that in md.c > > > > > > This sounds wrong. If a userspace process is waiting on > > > md_event_waiters then the md module is "busy" and the rmmod attempt > > > should fail? > > > > Al Viro says "no" quite firmly. > > > > I think the core argument is that > > > > rmmod md-mod < /proc/mdstat > > > > would deadlock. > > Well, only if the rmmod hangs around waiting for the module to go idle. > I'm thinking rmmod should fail. EBUSY. > > > http://marc.info/?l=linux-fsdevel&m=133024267507384 > > Why don't a billion blocking-procfs-read sites have this problem? > This issue of a file in /proc (actually /proc/fs/nfsd which is a separate filesystem) blocking came up on the NFS list recently. Experiments suggested that no other files in /proc block reads (though /proc/vmcore takes somewhat longer to 'cat' than most). So the offending file was changed to never block. i.e. I think your 'billion' estimate is a little high, by one billion. :-) NeilBrown
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