From fee9903a59c0d09032628e49a8c52dde54418452 Mon Sep 17 00:00:00 2001 From: Karl Williamson Date: Wed, 25 Nov 2020 18:20:28 -0700 Subject: [PATCH] Avoid deadlock with PERL_MEM_LOG This fixes GH #18341 The Perl wrapper for getenv() was changed in 5.32 to allocate memory to squirrel safely away the result of the wrapped getenv() call. It does this while in a critical section so as to make sure another thread can't interrupt it and destroy it. Unfortunately, when Perl is compiled for debugging memory problems and has PERL_MEM_LOG enabled, that allocation causes a recursive call to getenv() for the purpose of checking an environment variable to see how to log that allocation. And hence it deadlocks trying to enter the critical section. There are various solutions. One is to use or emulate a general semaphore instead of a binary one. This is effectively what PL_lc_numeric_mutex_depth does for another mutex, and the code for that could be used as a template. But given that this is an extreme edge case which requires Perl to be specially compiled to enable this feature which is used only for debugging, a much simpler, if less safe if it were to ever be used in production, solution should suffice. Tony Cook suggested just avoiding the wrapper for this particular purpose. --- util.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/util.c b/util.c index 5989a582c0e9..40e6b8dc1026 100644 --- a/util.c +++ b/util.c @@ -5008,7 +5008,11 @@ S_mem_log_common(enum mem_log_type mlt, const UV n, PERL_ARGS_ASSERT_MEM_LOG_COMMON; - pmlenv = PerlEnv_getenv("PERL_MEM_LOG"); + /* Use plain getenv() to avoid potential deadlock with PerlEnv_getenv(). + * This means that 'pmlenv' is not protected from other threads overwriting + * it on platforms where getenv() returns an internal static pointer. See + * GH #18341 */ + pmlenv = getenv("PERL_MEM_LOG"); if (!pmlenv) return; if (mlt < MLT_NEW_SV ? strchr(pmlenv,'m') : strchr(pmlenv,'s'))