On Fri, 2012-01-06 at 01:35 +0100, Jan Kara wrote: > On Thu 08-12-11 10:04:35, Kamal Mostafa wrote: > > From: Valerie Aurora <val@xxxxxxxxxxxxxxxxx> > > > > Use trylock in try_to_writeback_inodes_sb to avoid read-write > > deadlocks that could be triggered by freeze. > Christoph asked about what is the exact deadlock this patch tries to fix. > I don't think you answered that. So can you elaborate please? Is it somehow > connected with the fact that ext4 calls try_to_writeback_inodes_sb() with > i_mutex held? > > Honza This was discussed in the thread http://www.spinics.net/lists/linux-fsdevel/msg48754.html Summarizing... Jan> What's exactly the deadlock trylock protects from here? Jan> Or is it just an optimization? Val> The trylock is an optimization Dave Chinner suggested. The first Val> version I wrote acquired the lock and then checked vfs_is_frozen(). Dave> It's not so much an optimisation, but the general case of avoiding Dave> read-write deadlocks such that freezing can trigger. I think remount Dave> can trigger the same deadlock as freezing, so the trylock avoids Dave> both deadlock cases rather than just working around the freeze Dave> problem.... -Kamal > > BugLink: https://bugs.launchpad.net/bugs/897421 > > Signed-off-by: Valerie Aurora <val@xxxxxxxxxxxxxxxxx> > > Cc: Kamal Mostafa <kamal@xxxxxxxxxxxxx> > > Tested-by: Peter M. Petrakis <peter.petrakis@xxxxxxxxxxxxx> > > [kamal@xxxxxxxxxxxxx: patch restructure] > > Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx> > > --- > > fs/fs-writeback.c | 13 ++++++++----- > > 1 files changed, 8 insertions(+), 5 deletions(-) > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > > index ea89b3f..3a80f1b 100644 > > --- a/fs/fs-writeback.c > > +++ b/fs/fs-writeback.c > > @@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb); > > * try_to_writeback_inodes_sb - start writeback if none underway > > * @sb: the superblock > > * > > - * Invoke writeback_inodes_sb if no writeback is currently underway. > > - * Returns 1 if writeback was started, 0 if not. > > + * Invoke writeback_inodes_sb if no writeback is currently underway > > + * and no one else holds the s_umount lock. Returns 1 if writeback > > + * was started, 0 if not. > > */ > > int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason) > > { > > @@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb); > > * @sb: the superblock > > * @nr: the number of pages to write > > * > > - * Invoke writeback_inodes_sb if no writeback is currently underway. > > - * Returns 1 if writeback was started, 0 if not. > > + * Invoke writeback_inodes_sb if no writeback is currently underway > > + * and no one else holds the s_umount lock. Returns 1 if writeback > > + * was started, 0 if not. > > */ > > int try_to_writeback_inodes_sb_nr(struct super_block *sb, > > unsigned long nr, > > enum wb_reason reason) > > { > > if (!writeback_in_progress(sb->s_bdi)) { > > - down_read(&sb->s_umount); > > + if (!down_read_trylock(&sb->s_umount)) > > + return 0; > > if (nr == 0) > > writeback_inodes_sb(sb, reason); > > else > > -- > > 1.7.5.4 > >
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