In my previous post, I discussed a bug that brought up in code review, that bug made me go into near panic mode. Here is the issue:
In order to understand this bug, you have to take into account multiple concurrent threads at the same time. Look at the ComputeHashAndPutInCache() method, where we register an eviction callback for the item in the cache. When we evict the item, we return the buffer to the buffer pool.
We want to avoid allocating memory, so that is certainly something desirable, no? However, consider what happens if I have a thread in ComputeHash(), getting a value from the cache. Before I have a chance to look at the value, however, the cache will decide to evict it. At which point the eviction callback will run.
We’ll return the buffer back to the buffer pool, where it may be used again by something else. I am also using this buffer to do other things from the caller of the ComputeHash() call. This is a somewhat convoluted use after free issue, basically.
And I find is super scary bug because of its affects:
- Randomly, and rarely, some buffer will contain the wrong data, leading to wrong results (but hard to track it down).
- Trying to find such a bug after the fact, however, is nearly impossible.
- Most of the debugging techniques (repeating the operation for a particular value) will make it go away (the cache will keep the value and not evict it).
In short, this is a recipe for a really nasty debug session and an impossible to resolve bug. All from code that looks very much like an innocent bystander.
Now, I can obviously fix it by not using the array pool, but that may cause me to allocate more memory than I should. How would you approach fixing this issue?
More posts in "Challenge" series:
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- (30 Jun 2022) Find the stack smash bug…
- (03 Jun 2022) Spot the data corruption
- (06 May 2022) Spot the optimization–solution
- (05 May 2022) Spot the optimization
- (06 Apr 2022) Why is this code broken?
- (16 Dec 2021) Find the slow down–answer
- (15 Dec 2021) Find the slow down
- (03 Nov 2021) The code review bug that gives me nightmares–The fix
- (02 Nov 2021) The code review bug that gives me nightmares–the issue
- (01 Nov 2021) The code review bug that gives me nightmares
- (16 Jun 2021) Detecting livelihood in a distributed cluster
- (21 Apr 2020) Generate matching shard id–answer
- (20 Apr 2020) Generate matching shard id
- (02 Jan 2020) Spot the bug in the stream
- (28 Sep 2018) The loop that leaks–Answer
- (27 Sep 2018) The loop that leaks
- (03 Apr 2018) The invisible concurrency bug–Answer
- (02 Apr 2018) The invisible concurrency bug
- (31 Jan 2018) Find the bug in the fix–answer
- (30 Jan 2018) Find the bug in the fix
- (19 Jan 2017) What does this code do?
- (26 Jul 2016) The race condition in the TCP stack, answer
- (25 Jul 2016) The race condition in the TCP stack
- (28 Apr 2015) What is the meaning of this change?
- (26 Sep 2013) Spot the bug
- (27 May 2013) The problem of locking down tasks…
- (17 Oct 2011) Minimum number of round trips
- (23 Aug 2011) Recent Comments with Future Posts
- (02 Aug 2011) Modifying execution approaches
- (29 Apr 2011) Stop the leaks
- (23 Dec 2010) This code should never hit production
- (17 Dec 2010) Your own ThreadLocal
- (03 Dec 2010) Querying relative information with RavenDB
- (29 Jun 2010) Find the bug
- (23 Jun 2010) Dynamically dynamic
- (28 Apr 2010) What killed the application?
- (19 Mar 2010) What does this code do?
- (04 Mar 2010) Robust enumeration over external code
- (16 Feb 2010) Premature optimization, and all of that…
- (12 Feb 2010) Efficient querying
- (10 Feb 2010) Find the resource leak
- (21 Oct 2009) Can you spot the bug?
- (18 Oct 2009) Why is this wrong?
- (17 Oct 2009) Write the check in comment
- (15 Sep 2009) NH Prof Exporting Reports
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- (13 Mar 2009) C# Rewriting
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- (26 Jul 2008) Find the bug fix
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- (05 Jun 2008) why did the tests fail?
- (27 May 2008) Striving for better syntax
- (13 Apr 2008) calling generics without the generic type
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- (21 Jan 2008) Strongly typing weakly typed code
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