I can reproduce the hang with 
mpiexec -n 2 ./matmatmult

It seems in an infinite loop of calling MatDensePlaceArray() from 

#0  MatDensePlaceArray (mat=0xda5c50, array=0xd15e60)
    at /home/hongsu/soft/petsc/src/mat/impls/dense/mpi/mpidense.c:2047
#1  0x00007fa0d13bf4f7 in MatDenseGetSubMatrix_SeqDense (A=0xcfb2b0, cbegin=0,
    cend=0, v=0xd90370)
    at /home/hongsu/soft/petsc/src/mat/impls/dense/seq/dense.c:2997
#2  0x00007fa0d13c574e in MatDenseGetSubMatrix (A=0xcfb2b0, cbegin=0, cend=0,
    v=0xd90370) at /home/hongsu/soft/petsc/src/mat/impls/dense/seq/dense.c:3371
#3  0x00007fa0d13db5ce in MatDenseGetSubMatrix_MPIDense (A=0xca5250, cbegin=0,
    cend=0, v=0x7ffe87d41de0)
    at /home/hongsu/soft/petsc/src/mat/impls/dense/mpi/mpidense.c:1835
#4  0x00007fa0d13c574e in MatDenseGetSubMatrix (A=0xca5250, cbegin=0, cend=0,
    v=0x7ffe87d41de0)
    at /home/hongsu/soft/petsc/src/mat/impls/dense/seq/dense.c:3371
#5  0x00007fa0d179c2fa in MatMatMultNumeric_MPIAIJ_MPIDense (A=0xc55490,
    B=0xca5250, C=0xd282b0)
    at /home/hongsu/soft/petsc/src/mat/impls/aij/mpi/mpimatmatmult.c:593
#6  0x00007fa0d1181331 in MatProductNumeric_AB (mat=0xd282b0)
    at /home/hongsu/soft/petsc/src/mat/interface/matproduct.c:567
#7  0x00007fa0d1182c14 in MatProductNumeric (mat=0xd282b0)
    at /home/hongsu/soft/petsc/src/mat/interface/matproduct.c:679
#8  0x00007fa0d115ef69 in MatProduct_Private (A=0xc55490, B=0xca5250,
    scall=MAT_INITIAL_MATRIX, fill=-2, ptype=MATPRODUCT_AB, C=0x7ffe87d42018)
    at /home/hongsu/soft/petsc/src/mat/interface/matrix.c:9405
---Type <return> to continue, or q <return> to quit---
#9  0x00007fa0d115f274 in MatMatMult (A=0xc55490, B=0xca5250, scall=MAT_INITIAL_MATRIX, fill=-2,
    C=0x7ffe87d42018) at /home/hongsu/soft/petsc/src/mat/interface/matrix.c:9445
#10 0x000000000040130a in main (argc=2, argv=0x7ffe87d42108) at ex1.c:20

I'll try to figure out what is going on. If anyone has a clue, please help. The above stack comes from 'release' branch.
Hong
From: petsc-dev <petsc-dev-bounces@mcs.anl.gov> on behalf of Pierre Jolivet <pierre@joliv.et>
Sent: Sunday, February 28, 2021 4:17 PM
To: For users of the development version of PETSc <petsc-dev@mcs.anl.gov>
Subject: [petsc-dev] Infinite loop in A*B
 
Hello,
The following MWE loops indefinitely for MPI_Comm_size in {2; 3}.
Nothing fancy, just MatAIJ and MatDense.
The problem is either in MatMPIDenseScatter() or MatMatMultSymbolic_MPIAIJ_MPIDense(), I believe, so if someone familiar with those routines can figure out a hot fix, I’m all ears.
I could of course switch to a MatMult(), but the same infinite loop happens in another more complex code with
A = rows=8, cols=35212
B = rows=35212, cols=9
So I’ll need a fix eventually.

Thanks,
Pierre