Re: [PATCH v5 09/24] util: introduce some API docs for logging APIs

Daniel P. Berrangé <[email protected]> writes:

> 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.
>
> Reported-by: Markus Armbruster <[email protected]>
> Signed-off-by: Daniel P. Berrangé <[email protected]>
> ---
>  include/qemu/log-for-trace.h | 17 ++++++++++++++++-
>  include/qemu/log.h           | 31 +++++++++++++++++++++++++++++++
>  rust/util/src/log.rs         |  6 ++++++
>  3 files changed, 53 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) != 0;
>  }
>  
> -/* 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.

Note for later: "should normally".

> + *
> + * 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, ...);
>  
>  #endif
> diff --git a/include/qemu/log.h b/include/qemu/log.h
> index 7effba4da4..e9d3c6806b 100644
> --- a/include/qemu/log.h
> +++ b/include/qemu/log.h
> @@ -41,7 +41,38 @@ bool qemu_log_separate(void);
>  
>  /* Lock/unlock output. */
>  
> +/**
> + * 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 must only be used
> + * to output complete log messages.

"must".

> + *
> + * A typical usage pattern would be
> + *
> + *    FILE *f = 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);
>  
>  /* Logging functions: */
> diff --git a/rust/util/src/log.rs b/rust/util/src/log.rs
> index 0a4bc4249a..6a3a30d8d8 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 should only be used for emitting complete

"should only".

> +/// log messages. Where it is required to incrementally output string
> +/// fragments to construct a complete message, `LogGuard::new()` should
> +/// be directly used in combination with `writeln()` to avoid output
> +/// races with other QEMU threads.
>  /// ```
>  #[macro_export]
>  macro_rules! log_mask_ln {

"Should normally" suggests there are exceptions.

"Should only" does not.

"Must" is a bit stronger still.

Which of the three do we want?