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(cst-prg-67-174.cust.vodafone.cz. [46.135.67.174]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43d43f47c97sm23327415e9.14.2025.03.19.09.38.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 19 Mar 2025 09:38:33 -0700 (PDT) From: Mateusz Guzik To: brauner@kernel.org Cc: viro@zeniv.linux.org.uk, jack@suse.cz, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Mateusz Guzik Subject: [PATCH] fs: reduce work in fdget_pos() Date: Wed, 19 Mar 2025 17:38:25 +0100 Message-ID: <20250319163825.1847621-1-mjguzik@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 1. predict the file was found 2. explicitly compare the ref to "one", ignoring the dead zone The latter arguably improves the behavior to begin with. Suppose the count turned bad -- the previously used ref routine is going to check for it and return 0, indicating the count does not necessitate taking ->f_pos_lock. But there very well may be several users. i.e. not paying for special-casing the dead zone improves semantics. Sizes are as follows (in bytes; gcc 13, x86-64): stock: 316 likely(): 296 likely()+ref: 278 Signed-off-by: Mateusz Guzik --- fs/file.c | 5 +++-- include/linux/file_ref.h | 14 ++++++++++++++ 2 files changed, 17 insertions(+), 2 deletions(-) diff --git a/fs/file.c b/fs/file.c index ddefb5c80398..190463e61934 100644 --- a/fs/file.c +++ b/fs/file.c @@ -1193,7 +1193,8 @@ struct fd fdget_raw(unsigned int fd) static inline bool file_needs_f_pos_lock(struct file *file) { return (file->f_mode & FMODE_ATOMIC_POS) && - (file_count(file) > 1 || file->f_op->iterate_shared); + (__file_ref_read_raw(&file->f_ref) != FILE_REF_ONEREF || + file->f_op->iterate_shared); } bool file_seek_cur_needs_f_lock(struct file *file) @@ -1211,7 +1212,7 @@ struct fd fdget_pos(unsigned int fd) struct fd f = fdget(fd); struct file *file = fd_file(f); - if (file && file_needs_f_pos_lock(file)) { + if (likely(file) && file_needs_f_pos_lock(file)) { f.word |= FDPUT_POS_UNLOCK; mutex_lock(&file->f_pos_lock); } diff --git a/include/linux/file_ref.h b/include/linux/file_ref.h index 6ef92d765a66..7db62fbc0500 100644 --- a/include/linux/file_ref.h +++ b/include/linux/file_ref.h @@ -208,4 +208,18 @@ static inline unsigned long file_ref_read(file_ref_t *ref) return c >= FILE_REF_RELEASED ? 0 : c + 1; } +/* + * __file_ref_read_raw - Return the value stored in ref->refcnt + * @ref: Pointer to the reference count + * + * Return: The raw value found in the counter + * + * A hack for file_needs_f_pos_lock(), you probably want to use + * file_ref_read() instead. + */ +static inline unsigned long __file_ref_read_raw(file_ref_t *ref) +{ + return atomic_long_read(&ref->refcnt); +} + #endif