X-Git-Url: http://repo.macrolet.net/gitweb/?a=blobdiff_plain;f=doc%2Finternals%2Fsignals.texinfo;h=e5cfceb4d39d019f16376b173ad728eaebc9c94e;hb=ae09f8fd7765f6cab6ad317a13e27ff22ab0c11e;hp=1f27d3f014baa6b89a3d8a0ed5d98b07819aac92;hpb=dcf8b8ccc1e15a5c1c6aba00204b7d3a81827acc;p=sbcl.git

diff --git a/doc/internals/signals.texinfo b/doc/internals/signals.texinfo
index 1f27d3f..e5cfceb 100644
--- a/doc/internals/signals.texinfo
+++ b/doc/internals/signals.texinfo
@@ -73,20 +73,6 @@ is deferred by pseudo atomic and @code{WITHOUT-GCING}.
 @node Implementation warts
 @section Implementation warts
 
-@subsection RT signals
-
-Sending and receiving the same number of signals is crucial for
-@code{INTERRUPT-THREAD} and @code{sig_stop_for_gc}, hence they are
-real-time signals for which the kernel maintains a queue as opposed to
-just setting a flag for ``sigint pending''.
-
-Note, however, that the rt signal queue is finite and on current linux
-kernels a system wide resource. If the queue is full, SBCL tries to
-signal until it succeeds. This behaviour can lead to deadlocks, if a
-thread in a @code{WITHOUT-INTERRUPTS} is interrupted many times,
-filling up the queue and then a gc hits and tries to send
-@code{SIG_STOP_FOR_GC}.
-
 @subsection Miscellaneous issues
 
 Signal handlers should automatically restore errno and fp
@@ -159,9 +145,27 @@ derive the rule: in a @code{WITHOUT-GCING} form (or pseudo atomic for
 that matter) never wait for another thread that's not in
 @code{WITHOUT-GCING}.
 
+Somewhat of a special case, it is enforced by the runtime that
+@code{SIG_STOP_FOR_GC} and @code{SIG_RESUME_FROM_GC} always unblocked
+when we might trigger a gc (i.e. on alloc or calling into Lisp).
+
 @subsection Calling user code
 
 For the reasons above, calling user code, i.e. functions passed in, or
 in other words code that one cannot reason about, from non-reentrant
 code (holding locks), @code{WITHOUT-INTERRUPTS}, @code{WITHOUT-GCING}
 is dangerous and best avoided.
+
+@section Debugging
+
+It is not easy to debug signal problems. The best bet probably is to
+enable @code{QSHOW} and @code{QSHOW_SIGNALS} in runtime.h and once
+SBCL runs into problems attach gdb. A simple @code{thread apply all
+ba} is already tremendously useful. Another possibility is to send a
+SIGABORT to SBCL to provoke landing in LDB, if it's compiled with it
+and it has not yet done so on its own.
+
+Note, that fprintf used by QSHOW is not reentrant and at least on x86
+linux it is known to cause deadlocks, so place SHOW and co carefully,
+ideally to places where blockable signals are blocked. Use
+@code{QSHOW_SAFE} if you like.