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Date: Thu, 30 Nov 2017 02:40:38 +0100 From: "Rafael J. Wysocki" <rafael@...nel.org> To: Dave Chinner <david@...morbit.com> Cc: "Rafael J. Wysocki" <rafael@...nel.org>, "Luis R. Rodriguez" <mcgrof@...nel.org>, Al Viro <viro@...iv.linux.org.uk>, bart.vanassche@....com, ming.lei@...hat.com, "Ted Ts'o" <tytso@....edu>, "Darrick J. Wong" <darrick.wong@...cle.com>, Jiri Kosina <jikos@...nel.org>, "Rafael J. Wysocki" <rjw@...ysocki.net>, Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>, linux-fsdevel@...r.kernel.org, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, Todd Brandt <todd.e.brandt@...ux.intel.com>, nborisov@...e.com, Jan Kara <jack@...e.cz>, "Martin K. Petersen" <martin.petersen@...cle.com>, Oliver Neukum <ONeukum@...e.com>, oleksandr@...alenko.name, Oleg Antonyan <oleg.b.antonyan@...il.com>, Yu Chen <yu.chen.surf@...il.com>, Dan Williams <dan.j.williams@...el.com>, Linux PM <linux-pm@...r.kernel.org>, linux-block@...r.kernel.org, linux-xfs@...r.kernel.org, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 05/11] fs: add iterate_supers_excl() and iterate_supers_reverse_excl() On Thu, Nov 30, 2017 at 2:34 AM, Dave Chinner <david@...morbit.com> wrote: > On Thu, Nov 30, 2017 at 12:48:15AM +0100, Rafael J. Wysocki wrote: >> On Thu, Nov 30, 2017 at 12:23 AM, Luis R. Rodriguez <mcgrof@...nel.org> wrote: >> > There are use cases where we wish to traverse the superblock list >> > but also capture errors, and in which case we want to avoid having >> > our callers issue a lock themselves since we can do the locking for >> > the callers. Provide a iterate_supers_excl() which calls a function >> > with the write lock held. If an error occurs we capture it and >> > propagate it. >> > >> > Likewise there are use cases where we wish to traverse the superblock >> > list but in reverse order. The new iterate_supers_reverse_excl() helpers >> > does this but also also captures any errors encountered. >> > >> > Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org> >> > --- >> > fs/super.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> > include/linux/fs.h | 2 ++ >> > 2 files changed, 93 insertions(+) >> > >> > diff --git a/fs/super.c b/fs/super.c >> > index a63513d187e8..885711c1d35b 100644 >> > --- a/fs/super.c >> > +++ b/fs/super.c >> > @@ -605,6 +605,97 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg) >> > spin_unlock(&sb_lock); >> > } >> > >> > +/** >> > + * iterate_supers_excl - exclusively call func for all active superblocks >> > + * @f: function to call >> > + * @arg: argument to pass to it >> > + * >> > + * Scans the superblock list and calls given function, passing it >> > + * locked superblock and given argument. Returns 0 unless an error >> > + * occurred on calling the function on any superblock. >> > + */ >> > +int iterate_supers_excl(int (*f)(struct super_block *, void *), void *arg) >> > +{ >> > + struct super_block *sb, *p = NULL; >> > + int error = 0; >> > + >> > + spin_lock(&sb_lock); >> > + list_for_each_entry(sb, &super_blocks, s_list) { >> > + if (hlist_unhashed(&sb->s_instances)) >> > + continue; >> > + sb->s_count++; >> > + spin_unlock(&sb_lock); >> >> Can anything bad happen if the list is modified at this point by a >> concurrent thread? > > No. We have a valid reference to sb->s_count and that keeps it on > the list while we have the lock dropped. The sb reference isn't > dropped until we've iterated to the next sb on the list and taken a > reference to that, hence it's safe to drop and regain the list lock > without needing to restart the iteration. > >> > + >> > + down_write(&sb->s_umount); >> > + if (sb->s_root && (sb->s_flags & SB_BORN)) { >> > + error = f(sb, arg); >> > + if (error) { >> > + up_write(&sb->s_umount); >> > + spin_lock(&sb_lock); >> > + __put_super(sb); >> > + break; >> > + } >> > + } >> > + up_write(&sb->s_umount); >> > + >> > + spin_lock(&sb_lock); >> > + if (p) >> > + __put_super(p); >> > + p = sb; > > This code here is what drops the reference to the previous sb > we've iterated past. > > FWIW, this "hold until next is held" iteration pattern is used > frequently for inodes, dentries, and other reference counted VFS > objects so we can iterate the list without needing to hold the > list lock for the entire iteration.... OK, thanks!
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