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c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1770823587; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=quFtc628f984Q3yp25Wk6fR4lXqtSsXdbUO1vrf3FKQ=; b=XGum1ddFipUAH/hYvTHdJweaOPPGUFkBf4H8jZS9gREAgHI2pkosMQ5TowJrYQWluG3591 yx6V73SPi/Q3f+aW9MOjmMG9Mlj3VRsl+ekm14nB1y+UY/GY3qA+0qGhXyICAcMoOTm6JW dR3mVPHHCxiopfwnW/JvUsASRDD9TDw= X-MC-Unique: FVN3tseiN7q5m3q1l49CNA-1 X-Mimecast-MFC-AGG-ID: FVN3tseiN7q5m3q1l49CNA_1770823581 From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Cc: Manos Pitsidianakis , Stefan Weil , "Dr. David Alan Gilbert" , Pierrick Bouvier , devel@lists.libvirt.org, =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Hanna Reitz , Kevin Wolf , qemu-block@nongnu.org, qemu-rust@nongnu.org, Paolo Bonzini , Markus Armbruster , Gerd Hoffmann , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Christian Schoenebeck , Richard Henderson Subject: [PATCH v6 11/27] util: introduce some API docs for logging APIs Date: Wed, 11 Feb 2026 15:24:52 +0000 Message-ID: <20260211152508.732487-12-berrange@redhat.com> In-Reply-To: <20260211152508.732487-1-berrange@redhat.com> References: <20260211152508.732487-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1770823653915158500 There is a gotcha with qemu_log() usage in a threaded process. If fragments of a log message are output via qemu_log() it is possible for messages from two threads to get mixed up. To prevent this qemu_log_trylock() should be used, along with fprintf(f) calls. This is a subtle problem that needs to be explained in the API docs to ensure correct usage. In the Rust code, the log_mask_ln method which is conceptually equivalent to the C qemu_log() call will unconditionally append a newline so must only ever be used for complete log messages. Reported-by: Markus Armbruster Signed-off-by: Daniel P. Berrang=C3=A9 --- include/qemu/log-for-trace.h | 17 ++++++++++++++++- include/qemu/log.h | 32 ++++++++++++++++++++++++++++++++ rust/util/src/log.rs | 6 ++++++ 3 files changed, 54 insertions(+), 1 deletion(-) diff --git a/include/qemu/log-for-trace.h b/include/qemu/log-for-trace.h index f3a8791f1d..6861a1a4b7 100644 --- a/include/qemu/log-for-trace.h +++ b/include/qemu/log-for-trace.h @@ -29,7 +29,22 @@ static inline bool qemu_loglevel_mask(int mask) return (qemu_loglevel & mask) !=3D 0; } =20 -/* main logging function */ +/** + * qemu_log: report a log message + * @fmt: the format string for the message + * @...: the format string arguments + * + * This will emit a log message to the current output stream. + * + * The @fmt string should normally represent a complete line + * of text, and thus end with a newline character. + * + * While it is possible to incrementally output fragments of + * a complete line using qemu_log, this is inefficient and + * races with other threads. For outputting fragments it is + * strongly preferred to use the qemu_log_trylock() method + * combined with fprintf(). + */ void G_GNUC_PRINTF(1, 2) qemu_log(const char *fmt, ...); =20 #endif diff --git a/include/qemu/log.h b/include/qemu/log.h index 7effba4da4..b49a1a5fe3 100644 --- a/include/qemu/log.h +++ b/include/qemu/log.h @@ -41,7 +41,39 @@ bool qemu_log_separate(void); =20 /* Lock/unlock output. */ =20 +/** + * Acquires a lock on the current log output stream. + * The returned FILE object should be used with the + * fprintf() function to output the log message, and + * then qemu_log_unlock() called to release the lock. + * + * The primary use case is to be able to incrementally + * output fragments of a complete log message in an + * efficient and race free manner. + * + * The simpler qemu_log() method should normally only + * be used to output complete log messages, and not + * within scope of a qemu_log_trylock() call. + * + * A typical usage pattern would be + * + * FILE *f =3D qemu_log_trylock() + * + * fprintf(f, "Something "); + * fprintf(f, "Something "); + * fprintf(f, "Something "); + * fprintf(f, "The end\n"); + * + * qemu_log_unlock(f); + * + * Returns: the current FILE if available, NULL on error + */ FILE *qemu_log_trylock(void) G_GNUC_WARN_UNUSED_RESULT; + +/** + * Releases the lock on the log output, previously + * acquired by qemu_log_trylock(). + */ void qemu_log_unlock(FILE *fd); =20 /* Logging functions: */ diff --git a/rust/util/src/log.rs b/rust/util/src/log.rs index 0a4bc4249a..a85db804a7 100644 --- a/rust/util/src/log.rs +++ b/rust/util/src/log.rs @@ -134,6 +134,12 @@ fn drop(&mut self) { /// "Address 0x{:x} out of range", /// error_address, /// ); +/// +/// The `log_mask_ln` macro must only be used for emitting complete +/// log messages. Where it is required to incrementally output string +/// fragments to construct a complete message, `LogGuard::new()` must +/// be directly used in combination with `writeln()` to avoid output +/// races with other QEMU threads. /// ``` #[macro_export] macro_rules! log_mask_ln { --=20 2.53.0