From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-eopbgr10117.outbound.protection.outlook.com ([40.107.1.117]:59552 "EHLO EUR02-HE1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726929AbeI0Oyv (ORCPT ); Thu, 27 Sep 2018 10:54:51 -0400 Subject: Re: [PATCH 6/6] fuse: Do not take fuse_conn::lock on fuse_request_send_background() From: Kirill Tkhai To: Miklos Szeredi Cc: linux-fsdevel@vger.kernel.org References: <153538208536.18303.10732945923322972743.stgit@localhost.localdomain> <153538379617.18303.11871598131511120870.stgit@localhost.localdomain> <20180926122537.GC23439@veci.piliscsaba.redhat.com> <6978de82-7681-144a-318c-daadce00c230@virtuozzo.com> Message-ID: <44bc25a9-aed7-57a7-3a17-06fc8f02ab51@virtuozzo.com> Date: Thu, 27 Sep 2018 11:37:34 +0300 MIME-Version: 1.0 In-Reply-To: <6978de82-7681-144a-318c-daadce00c230@virtuozzo.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Hi, Miklos, should I resend the series with the patch you changed, or you are already taken it since there is your SoB? Thanks, Kirill On 26.09.2018 18:18, Kirill Tkhai wrote: > On 26.09.2018 15:25, Miklos Szeredi wrote: >> On Mon, Aug 27, 2018 at 06:29:56PM +0300, Kirill Tkhai wrote: >>> Currently, we take fc->lock there only to check for fc->connected. >>> But this flag is changed only on connection abort, which is very >>> rare operation. Good thing looks to make fuse_request_send_background() >>> faster, while fuse_abort_conn() slowler. >>> >>> So, we make fuse_request_send_background() lockless and mark >>> (fc->connected == 1) region as RCU-protected. Abort function >>> just uses synchronize_sched() to wait till all pending background >>> requests is being queued, and then makes ordinary abort. >>> >>> Note, that synchronize_sched() is used instead of synchronize_rcu(), >>> since we want to check for fc->connected without rcu_dereference() >>> in fuse_request_send_background() (i.e., not to add memory barriers >>> to this hot path). >> >> Apart from the inaccuracies in the above (_sched variant is for scheduling and >> NMI taking code; _sched variant requires rcu_dereference() as well; >> rcu_dereference() does not add barriers; rcu_dereference() is only for pointers, >> so we can't use it for an integer), > > Writing this I was inspired by expand_fdtable(). Yes, the description confuses, > and we don't need rcu_dereference() since we do not touch memory pointed by __rcu > pointer, we have no pointer at all. synchronize_sched() guarantees: > > On systems with more than one CPU, when synchronize_sched() returns, > each CPU is guaranteed to have executed a full memory barrier since the > end of its last RCU-sched read-side critical section whose beginning > preceded the call to synchronize_sched(). > > (and rcu_dereference() unfolds in smp_read_barrier_depends(), which I mean as > added barriers) > > But it does not so matter. I'm OK with the patch you updated. > >> wouldn't it be simpler to just use bg_lock >> for checking ->connected, and lock bg_lock (as well as fc->lock) when setting >> ->connected? >> >> Updated patch below (untested). > > Tested it. Works for me. > > Thanks, > Kirill > >> >> --- >> Subject: fuse: do not take fc->lock in fuse_request_send_background() >> From: Kirill Tkhai >> Date: Mon, 27 Aug 2018 18:29:56 +0300 >> >> Currently, we take fc->lock there only to check for fc->connected. >> But this flag is changed only on connection abort, which is very >> rare operation. >> >> Signed-off-by: Kirill Tkhai >> Signed-off-by: Miklos Szeredi >> --- >> fs/fuse/dev.c | 46 +++++++++++++++++++++++----------------------- >> fs/fuse/file.c | 4 +++- >> fs/fuse/fuse_i.h | 4 +--- >> 3 files changed, 27 insertions(+), 27 deletions(-) >> >> --- a/fs/fuse/dev.c >> +++ b/fs/fuse/dev.c >> @@ -574,42 +574,38 @@ ssize_t fuse_simple_request(struct fuse_ >> return ret; >> } >> >> -/* >> - * Called under fc->lock >> - * >> - * fc->connected must have been checked previously >> - */ >> -void fuse_request_send_background_nocheck(struct fuse_conn *fc, >> - struct fuse_req *req) >> +bool fuse_request_queue_background(struct fuse_conn *fc, struct fuse_req *req) >> { >> - BUG_ON(!test_bit(FR_BACKGROUND, &req->flags)); >> + bool queued = false; >> + >> + WARN_ON(!test_bit(FR_BACKGROUND, &req->flags)); >> if (!test_bit(FR_WAITING, &req->flags)) { >> __set_bit(FR_WAITING, &req->flags); >> atomic_inc(&fc->num_waiting); >> } >> __set_bit(FR_ISREPLY, &req->flags); >> spin_lock(&fc->bg_lock); >> - fc->num_background++; >> - if (fc->num_background == fc->max_background) >> - fc->blocked = 1; >> - if (fc->num_background == fc->congestion_threshold && fc->sb) { >> - set_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC); >> - set_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC); >> + if (likely(fc->connected)) { >> + fc->num_background++; >> + if (fc->num_background == fc->max_background) >> + fc->blocked = 1; >> + if (fc->num_background == fc->congestion_threshold && fc->sb) { >> + set_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC); >> + set_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC); >> + } >> + list_add_tail(&req->list, &fc->bg_queue); >> + flush_bg_queue(fc); >> + queued = true; >> } >> - list_add_tail(&req->list, &fc->bg_queue); >> - flush_bg_queue(fc); >> spin_unlock(&fc->bg_lock); >> + >> + return queued; >> } >> >> void fuse_request_send_background(struct fuse_conn *fc, struct fuse_req *req) >> { >> - BUG_ON(!req->end); >> - spin_lock(&fc->lock); >> - if (fc->connected) { >> - fuse_request_send_background_nocheck(fc, req); >> - spin_unlock(&fc->lock); >> - } else { >> - spin_unlock(&fc->lock); >> + WARN_ON(!req->end); >> + if (!fuse_request_queue_background(fc, req)) { >> req->out.h.error = -ENOTCONN; >> req->end(fc, req); >> fuse_put_request(fc, req); >> @@ -2112,7 +2108,11 @@ void fuse_abort_conn(struct fuse_conn *f >> struct fuse_req *req, *next; >> LIST_HEAD(to_end); >> >> + /* Background queuing checks fc->connected under bg_lock */ >> + spin_lock(&fc->bg_lock); >> fc->connected = 0; >> + spin_unlock(&fc->bg_lock); >> + >> fc->aborted = is_abort; >> fuse_set_initialized(fc); >> list_for_each_entry(fud, &fc->devices, entry) { >> --- a/fs/fuse/fuse_i.h >> +++ b/fs/fuse/fuse_i.h >> @@ -863,9 +863,7 @@ ssize_t fuse_simple_request(struct fuse_ >> * Send a request in the background >> */ >> void fuse_request_send_background(struct fuse_conn *fc, struct fuse_req *req); >> - >> -void fuse_request_send_background_nocheck(struct fuse_conn *fc, >> - struct fuse_req *req); >> +bool fuse_request_queue_background(struct fuse_conn *fc, struct fuse_req *req); >> >> /* Abort all requests */ >> void fuse_abort_conn(struct fuse_conn *fc, bool is_abort); >> --- a/fs/fuse/file.c >> +++ b/fs/fuse/file.c >> @@ -1487,6 +1487,7 @@ __acquires(fc->lock) >> struct fuse_inode *fi = get_fuse_inode(req->inode); >> struct fuse_write_in *inarg = &req->misc.write.in; >> __u64 data_size = req->num_pages * PAGE_SIZE; >> + bool queued; >> >> if (!fc->connected) >> goto out_free; >> @@ -1502,7 +1503,8 @@ __acquires(fc->lock) >> >> req->in.args[1].size = inarg->size; >> fi->writectr++; >> - fuse_request_send_background_nocheck(fc, req); >> + queued = fuse_request_queue_background(fc, req); >> + WARN_ON(!queued); >> return; >> >> out_free: >>