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Date: Fri, 2 Feb 2024 12:02:19 +1100
From: David Gibson <david@gibson.dropbear.id.au>
To: Stefano Brivio <sbrivio@redhat.com>
Subject: Re: [PATCH] log: setlogmask(0) can actually result in a system call,
don't use it
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On Fri, Feb 02, 2024 at 12:32:57AM +0100, Stefano Brivio wrote:
> Before commit 32d07f5e59f2 ("passt, pasta: Completely avoid dynamic
> memory allocation"), we didn't store the current log mask in a
> variable, and we fetched it using setlogmask(0) wherever needed.
>=20
> But after that commit, we can just use our log_mask copy instead. And
> we should: with recent glibc versions, setlogmask(0) actually results
> in a system call, which causes a substantial overhead with high
> transfer rates: we use setlogmask(0) even to decide we don't want to
> print debug messages.
>=20
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
The basic idea looks fine to me, but there's a curly edge case. At
present vlogmsg() works ok before __openlog(), sending the message to
stderr. We rely on this because there are die() calls before our
__openlog(), which go via err() -> vlogmsg().
With this change, before the first __setlogmask(), which occurs right
after the __openlog() then log_mask is initialized to zero. Which
means, AFAICT, that we'll suppress all error messages before that
point, which isn't great.
> ---
> log.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>=20
> diff --git a/log.c b/log.c
> index 4a70e29..a66ecb7 100644
> --- a/log.c
> +++ b/log.c
> @@ -45,33 +45,33 @@ static time_t log_start; /* Start timestamp */
> int log_trace; /* --trace mode enabled */
> int log_to_stdout; /* Print to stdout instead of stderr */
> =20
> -#define BEFORE_DAEMON (setlogmask(0) =3D=3D LOG_MASK(LOG_EMERG))
> +#define BEFORE_DAEMON (log_mask =3D=3D LOG_MASK(LOG_EMERG))
> =20
> void vlogmsg(int pri, const char *format, va_list ap)
> {
> FILE *out =3D log_to_stdout ? stdout : stderr;
> struct timespec tp;
> =20
> - if (setlogmask(0) & LOG_MASK(LOG_DEBUG) && log_file =3D=3D -1) {
> + if (log_mask & LOG_MASK(LOG_DEBUG) && log_file =3D=3D -1) {
> clock_gettime(CLOCK_REALTIME, &tp);
> fprintf(out, "%lli.%04lli: ",
> (long long int)tp.tv_sec - log_start,
> (long long int)tp.tv_nsec / (100L * 1000));
> }
> =20
> - if ((LOG_MASK(LOG_PRI(pri)) & log_mask) || BEFORE_DAEMON) {
> + if ((log_mask & LOG_MASK(LOG_PRI(pri))) || BEFORE_DAEMON) {
> va_list ap2;
> =20
> va_copy(ap2, ap); /* Don't clobber ap, we need it again */
> if (log_file !=3D -1)
> logfile_write(pri, format, ap2);
> - else if (!(setlogmask(0) & LOG_MASK(LOG_DEBUG)))
> + else if (!(log_mask & LOG_MASK(LOG_DEBUG)))
> passt_vsyslog(pri, format, ap2);
> =20
> va_end(ap2);
> }
> =20
> - if ((setlogmask(0) & LOG_MASK(LOG_DEBUG) && log_file =3D=3D -1) ||
> + if ((log_mask & LOG_MASK(LOG_DEBUG) && log_file =3D=3D -1) ||
> (BEFORE_DAEMON && !(log_opt & LOG_PERROR))) {
> (void)vfprintf(out, format, ap);
> if (format[strlen(format)] !=3D '\n')
--=20
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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