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Wed, 21 Feb 2024 13:15:43 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap2.dmz-prg2.suse.org with ESMTPSA id g0CiGH/31WVHOwAAn2gu4w (envelope-from ); Wed, 21 Feb 2024 13:15:43 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 0E782A0807; Wed, 21 Feb 2024 14:15:39 +0100 (CET) Date: Wed, 21 Feb 2024 14:15:39 +0100 From: Jan Kara To: Chuck Lever Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, hughd@google.com, akpm@linux-foundation.org, Liam.Howlett@oracle.com, oliver.sang@intel.com, feng.tang@intel.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, maple-tree@lists.infradead.org, linux-mm@kvack.org, lkp@intel.com Subject: Re: [PATCH v2 1/6] libfs: Re-arrange locking in offset_iterate_dir() Message-ID: <20240221131539.wmr5xde7xode2xn5@quack3> References: <170820083431.6328.16233178852085891453.stgit@91.116.238.104.host.secureserver.net> <170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <170820142021.6328.15047865406275957018.stgit@91.116.238.104.host.secureserver.net> X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 4A7FB8000E X-Stat-Signature: wrbkn3q1ffqhj1k5k98bykg46eczf6k4 X-HE-Tag: 1708521345-241860 X-HE-Meta: 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 d1itUNa8 9Cf3W9wBqGXUxlRiADUvkOalYgCC0Vsbuqh/5Bj8VaN+afWxdR1MprGlB7DCFU87aDwghqE726RCfjm86HWkBEaBBuwHbc/+/WDsXXCFm8al9xGjADngzfryjNmoS2GFMsqEKWfGbrtzb3ZuM/JESVQq8+rLquDrYqtA/5irUkLB1ajfkQCLa/Xqw4M9TMxRjz/tj0y8OKRTvu6C0Phmam2X4xBeEKcMOo2nr79icZNSPR8txJY/H1nEQB7eCO2TtJAtw13/QaWvKB2LDakleG7AgJ7NpB0Srcw//MwiAOWi5he/GqsFOWSea0+xxLeeY9fSKWbfHwhtpOIv+WQcENsxXsY4OEyeFQHEu X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat 17-02-24 15:23:40, Chuck Lever wrote: > From: Chuck Lever > > Liam and Matthew say that once the RCU read lock is released, > xa_state is not safe to re-use for the next xas_find() call. But the > RCU read lock must be released on each loop iteration so that > dput(), which might_sleep(), can be called safely. > > Thus we are forced to walk the offset tree with fresh state for each > directory entry. xa_find() can do this for us, though it might be a > little less efficient than maintaining xa_state locally. > > We believe that in the current code base, inode->i_rwsem provides > protection for the xa_state maintained in > offset_iterate_dir(). However, there is no guarantee that will > continue to be the case in the future. > > Since offset_iterate_dir() doesn't build xa_state locally any more, > there's no longer a strong need for offset_find_next(). Clean up by > rolling these two helpers together. > > Suggested-by: Liam R. Howlett > Message-ID: <170785993027.11135.8830043889278631735.stgit@91.116.238.104.host.secureserver.net> > Signed-off-by: Chuck Lever Looks good. Feel free to add: Reviewed-by: Jan Kara Honza > --- > fs/libfs.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/fs/libfs.c b/fs/libfs.c > index eec6031b0155..752e24c669d9 100644 > --- a/fs/libfs.c > +++ b/fs/libfs.c > @@ -402,12 +402,13 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence) > return vfs_setpos(file, offset, U32_MAX); > } > > -static struct dentry *offset_find_next(struct xa_state *xas) > +static struct dentry *offset_find_next(struct offset_ctx *octx, loff_t offset) > { > struct dentry *child, *found = NULL; > + XA_STATE(xas, &octx->xa, offset); > > rcu_read_lock(); > - child = xas_next_entry(xas, U32_MAX); > + child = xas_next_entry(&xas, U32_MAX); > if (!child) > goto out; > spin_lock(&child->d_lock); > @@ -430,12 +431,11 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry) > > static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx) > { > - struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode); > - XA_STATE(xas, &so_ctx->xa, ctx->pos); > + struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode); > struct dentry *dentry; > > while (true) { > - dentry = offset_find_next(&xas); > + dentry = offset_find_next(octx, ctx->pos); > if (!dentry) > return ERR_PTR(-ENOENT); > > @@ -444,8 +444,8 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx) > break; > } > > + ctx->pos = dentry2offset(dentry) + 1; > dput(dentry); > - ctx->pos = xas.xa_index + 1; > } > return NULL; > } > > -- Jan Kara SUSE Labs, CR